The subject invention is directed toward the art of heating, and more particularly, to a space heating system.
The invention is particularly suited for use as an air heating system for residential space heating requirements and will be described with particular reference thereto; however, as will become apparent, the invention is capable of broader application and could be used by many different heating systems.
The most common residential air heating system comprises a furnace including a heat exchanger heated by a single stage gas burner. A simple on-off, electrically operated room thermostat controls the firing of the burner. The air to be heated is conducted through the heat exchanger and to the space by an electric motor-driven fan which is operated whenever the burner is firing and furnace bonnet temperature commands or, alternatively, continuously driven to maintain a constant air circulation.
So long as fuel costs were relatively low, the prior art heating systems were acceptable. However, with recent fuel shortages and drastically increasing fuel costs, the low seasonal efficiency and other problems or shortcomings of these systems have made them comparatively undesirable.
To explain, it should be understood that the systems were normally substantially oversized in terms of their maximum heating capacity. This was primarily the result of conservatism on the part of architects and builders. Because of the oversizing, the systems operated far below their peak throughout the majority of the heating season. This type of operation was extremely inefficient because it produced frequent cycling which resulted in excessive losses in the form of heated air and combustion products lost up the chimney. During 90% of the heating season less than 50% of design capacity is required. Modulation, therefore, is the key to high seasonal efficiency. A heating surface that is large enough for design capacity can, with modulation, obtain maximum efficiency at lower ranges provided secondary air is also modulated.
An additional problem and a cause of substantial inefficiency was the electric motor-driven fan. In the typical installation, the fan was sized for maximum heating requirements. As noted earlier, this generally represents only a minor portion of the heating season. Consequently, the systems had a high electric power cost for the motor and a high fan noise level. In addition, the frequent cycling produced intermittent drafts within the residence.
Other typical disadvantages and problems with the described prior art systems included: inability to operate during an electric power failure; "scorching" of the heated air (causing carbonized dust deposits in heated space); relatively large physical size; the need for electrical power and control wiring; and, generally noisy operation.